Recently, in regard to a working medium used in a refrigerating cycle device, a conventional CFC refrigerant or an HCFC refrigerant which is considered to impart a harmful influence to an ozone layer is being replaced with an HFC refrigerant or an HC refrigerant whose ozone destruction coefficient is zero, wherein the HFC refrigerant or the HC refrigerant is used as a substitute refrigerant.
However, the HFC refrigerant has, as its material property, a drawback of having a large earth warming coefficient, while the HG refrigerant has a drawback that the refrigerant is strongly flammable although its earth warming coefficient is small. Further, an ammonium refrigerant which has been used conventionally has a drawback that the refrigerant is strongly flammable and has toxicity although the earth warming coefficient is zero.
Accordingly, an attention is focused on a CO2 refrigerant which exhibits substantially no earth warming coefficient as a material, is nonflammable, nontoxic and can be manufactured at a low cost. However, the CO2 refrigerant has a low critical temperature of 31.1° C. and hence, the condensation of the CO2 refrigerant is not generated at a high pressure side of a usual refrigerating cycle device.
In view of the above, as shown in FIG. 19, in Japanese Patent Publication 2132329 (Japanese Examined Patent Application Publication 7-18602), by providing an internal heat exchanger 103 which performs heat exchange between an outlet line of a cooler 102 at the high pressure side and a suction line of a compressor 101, the outlet of the cooler 102 is supercooled, while a low pressure receiver 106 is provided as capacity control means which adjusts an amount of refrigerant. The entire disclosure of Japanese Patent Publication 2132329 (Japanese Examined Patent Application Publication 7-18602) is incorporated herein by reference in its entirety.
Further, with respect to a room air conditioner or a car air conditioner for space heating and cooling, an indoor-side heat exchanger is required to be miniaturized, while an outdoor-side heat exchanger is large-sized compared to the indoor-side heat exchanger to satisfy energy saving at the time of space cooling attributed to the enhancement of condensing capacity and the enhancement of capacity at the time of space heating attributed to the enhancement of the heat absorbing capacity. Accordingly, an optimum refrigerant quantity which enables an operation with high efficiency at the cooling operation in which the outdoor-side heat exchanger having a large volume assumes a high-pressure side and the condensing of refrigerant having high density is performed becomes larger than an optimum refrigerant quantity at the time of heating operation and hence, it is effective to use a receiver which also substantially performs a buffer function.
Further, in Japanese Patent Publication 2931668, as shown in FIG. 19, to minimize the energy consumption of the device for a given capacity demand, the degree of opening of a throttle valve 4 is adjusted in accordance with a predetermined value.
That is, as shown in FIG. 20, when a refrigerating cycle having a high pressure of P is changed to a refrigerating cycle having a high pressure of P1, the increase of enthalpy difference of a refrigerating capacity Q becomes higher than the increase of enthalpy difference of an input W and hence, COP is increased. However, when the refrigerating cycle having the high pressure of P1 is changed to a refrigerating cycle having a high pressure of P2, the increase of the enthalpy difference of the refrigerating capacity Q reversely becomes smaller than the increase of the enthalpy difference of the input W and hence, the COP is lowered. That is, as indicated by the refrigerating cycle having the high pressure of P1 shown in FIG. 20, the CO2 refrigerant has a high pressure at which the COP becomes maximum theoretically.
Further, the heat pump cycle COP is a cycle which is obtained by adding 1 to the refrigerating cycle COP and hence, also in a case of the heat pump cycle, a value of the high pressure which ensures the maximum COP (hereinafter referred to as high-side pressure) becomes equal to a value of the high pressure of the refrigerating cycle.
The refrigerating cycle shown in FIG. 19 can be used as, for example, a space cooling device.
However, the provision of the receiver at low pressure brings about drawbacks such as the increase of cost and the increase of volume. To take a fact that, in an actual-use operation range, the pressure of the CO2 refrigerant becomes extremely high compared to the HCFC refrigerant and the HFC refrigerant which are used in the conventional refrigerating cycle device into consideration, the pressure resistance design for ensuring the safety becomes more severe. Particularly, with respect to air conditioning of a car, the further reduction in volume and the further reduction of weight are requested.
Further, in general, with respect to a device which performs space cooling, space heating/dehumidifying, it is necessary to compress a refrigerant at a pressure which is higher than a pressure necessary for a cooling device. Further, a temperature of the refrigerant compressed by a compressor in the device which performs space heating/dehumidifying becomes higher than a corresponding pressure in the device which performs space cooling.
That is, when a device is used as a dehumidifying device which performs cooling, heating or dehumidifying by adding a hot water cycle to the conventional refrigerating cycle in FIG. 19, it is necessary to operate the device at a higher-side pressure. Further, a temperature of a radiator is elevated and a compression ratio is increased.
Accordingly, when a device is used as a dehumidifier which performs cooling, heating or dehumidifying by adding the hot water cycle to the conventional refrigerating cycle shown in FIG. 19, following drawbacks arise.
That is, when the temperature of a radiator is high, that is, when an ambient temperature of the radiator is high or a small-sized radiator is used, the operation of the device at the high-side pressure which is considered to minimize the energy consumption increases the compression ratio and hence, there arise drawbacks that the efficiency of a compressor is largely lowered and the reliability of the compressor may be deteriorated. Further, since the high-side pressure is high, the pressure resistance design for ensuring the safety becomes more severe.
Further, a refrigerant holding quantity at the high pressure side of the refrigerating cycle device differs between time for space heating/dehumidifying and time for space cooling and hence, an imbalance is generated with respect to the optimum refrigerant quantity. Accordingly, it is necessary to eliminate the imbalance of the refrigerant quantity between time for space cooling and time for space heating/dehumidifying by adjusting the refrigerant holding quantity in the first heat exchanger 13 by changing middle pressure.